1. Field of the Invention
The present invention relates to a method of producing a magneto-resistive tunnel junction head for reading the magnetic field intensity from a magnetic recording medium or the like as a signal. In particular, the present invention relates to a method of producing a magneto-resistive tunnel junction head in which a higher output can be obtained for allowing the application of the head to ultra-high density magnetic recording.
2. Description of the Related Art
MR sensors based on the anisotropic magneto-resistance (AMR) or spin-valve (SV) effect are widely known and extensively used as read transducers in magnetic recording. MR sensors can probe the magnetic stray field coming out from transitions recorded on a recording medium by the resistance changes of a reading portion formed of magnetic materials. AMR sensors have quite a low resistance change ratio xcex94R/R, typically from 1 to 3%, whereas the SV sensors have a xcex94R/R ranging from 2 to 7% for the same magnetic field excursion. The SV magnetic read heads showing such high sensitivity are progressively supplanting the AMR read heads to achieve very high recording density, namely over several Giga bits per square inch (Gbits/in2).
Recently, a new MR sensor has attracted attention for its application potential in ultra-high density recording. Magneto-resistive tunnel junctions (MRJT, or synonymously referred to as TMR) are reported to have shown a resistance change ratio xcex94R/R over 12%. Although it has been expected that TMR sensors replace SV sensors in the near future as the demand for ultra-high density is ever growing, an application to the field of the magnetic heads has just started, and one of the outstanding objects is to develop a new head structure which can maximize the TMR properties. Great efforts of developments are still needed to design a new head structure since TMR sensors operate in CPP (Current Perpendicular to the Plane) geometry, which means that TMR sensors requires the current to flow in a thickness direction of a laminate film.
In a basic SV sensor which has been developed for practical applications, two ferromagnetic layers are separated by a non-magnetic layer, as described in U.S. Pat. No. 5,159,513. An exchange layer (FeMn) is further provided so as to be adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent ferromagnetic layer are exchange-coupled so that the magnetization of the ferromagnetic layer is strongly pinned (fixed) in one direction. The other ferromagnetic layer has its magnetization which is free to rotate in response to a small external magnetic field. When the magnetization of the ferromagnetic layers are changed from a parallel to an antiparallel configuration, the sensor resistance increases and a xcex94R/R in the range of 2 to 7% is observed.
In comparison between the SV sensor and the TMR sensor, the structure of the TMR is similar to the SV sensor except that the non-magnetic layer separating the two ferromagnetic layers is replaced by a tunnel barrier layer being an insulating layer and that the sense current flows perpendicular to the surfaces of the ferromagnetic layers. In the TMR sensor, the sense current flowing through the tunnel barrier layer is strongly dependent upon a spin-polarization state of the two ferromagnetic layers. When the magnetization of the two ferromagnetic layers are antiparallel to each other, the probability of the tunnel current is lowered, so that a high junction resistance is obtained. On the contrary, when the magnetization of the two ferromagnetic layers are parallel to each other, the probability of the tunnel current is heightened and thus a low junction resistance is obtained. The inventors of the present invention have attempted to design TMR heads the constructions of which are similar to those of SV heads. One of these head constructions is shown in FIG. 5. The TMR head 100 shown in FIG. 5 comprises a TMR element 200 having a laminate structure composed of a ferromagnetic free layer 120, a tunnel barrier layer 130, a ferromagnetic pinned layer 140, and an antiferromagnetic pinning layer 150. Insulating layers 191 and 191 are externally formed on the opposite ends (left and right directions of the drawing paper) of the TMR element 200. The ferromagnetic pinned layer 140 is pinned such that its magnetization direction is fixed in one direction (a depth direction of the drawing sheet), and the ferromagnetic free layer 120 can change its magnetization direction freely in response to an external signal magnetic field.
Biasing layers 161 and 161, for applying a bias magnetic field in the direction of the arrow (xcex1), are formed on the upper surface of both ends of the ferromagnetic free layer 120, which is disposed at an upper portion of the TMR element 200. The biasing layers 161 and 161 are composed of permanent magnet, for example. Thus, at portions where the biasing layers 161 contact with the upper surface of the ferromagnetic free layer 120, the magnetization direction of the ferromagnetic free layer 120 is pinned in the direction of the arrow (xcex1) by the exchange coupling magnetic field. In FIG. 5, numerals 171, 175 represent a pair of upper and the lower electrodes, and numerals 181, 185 represent a pair of upper and the lower shield layers.
It was confirmed that an effective bias magnetic field was applied to the ferromagnetic free layer 120 by employing the head construction shown in FIG. 5. However, the present inventors found that the following problems to be solved were raised in the head construction shown in FIG. 5.
Specifically, the TMR effect is a phenomenon that when a current is applied in a laminate direction between a pair of ferromagnetic layers (a ferromagnetic pinned layer and a ferromagnetic free layer) sandwiching a tunnel barrier layer therebetween, a tunnel current flowing in the tunnel barrier layer changes depending on a relative angle of magnetization between the ferromagnetic layers. The tunnel barrier layer is a thin insulation film which allows electrons to pass therethrough while keeping spin due to the magneto-resistive tunnel junction effect.
Therefore, as shown in FIG. 4A, when the ferromagnetic pinned layer and the ferromagnetic free layer are parallel in magnetization to each other, the tunneling probability is increased so that the resistance to current flowing therebetween is decreased (resistance value Rp).
In contrast, as shown in FIG. 4C, when both ferromagnetic layers are antiparallel in magnetization to each other, the tunneling probability is lowered, thus, the resistance to current flowing therebetween is increased (resistance value Rap).
In the intermediate state between the state shown in FIG. 4A and the state shown in FIG. 4C, i.e. when both ferromagnetic layers are orthogonal in magnetization to each other, a resistance value Rm takes a value between the resistance value Rp and the resistance value Rap so that a relation of Rp less than Rm less than Rap is satisfied.
It was found through experiments implemented by the present inventors that an unfavorable phenomenon as shown in FIGS. 6A and 6B was generated between the ferromagnetic pinned layer and the ferromagnetic free layer in the head structure shown in FIG. 5. Specifically, as shown in FIG. 6A, when the magnetization directions of the ferromagnetic pinned layer 140 and the free layer 120 are basically parallel to each other, magnetization in both end portions 120a and 120a of the free layer 120 is fixed in the direction of arrow xcex1 due to the exchange-coupling relative to the bias layers as described above. If a sense current i is caused to flow in the laminate direction in this state, the current mainly flows at the center portions of the layers where the magnetization directions are parallel to each other and thus the resistance is small. The total resistance value at this time is given by Rxe2x80x2p. On the other hand, as shown in FIG. 6B, when the magnetization directions of the ferromagnetic pinned layer 140 and the free layer 120 are basically antiparallel to each other (also in this case, the magnetization in the end portions 120a and 120a of the free layer 120 is fixed in the direction of arrow a due to the exchange-coupling relative to the bias layers as described above), if a sense current i is caused to flow in the laminate direction, the current does not mainly flow at the antiparallel center portions of the layers, but branches to mainly flow at both end portions where the resistance is small (currents is and is). The total resistance value in FIG. 6B is given by Rxe2x80x2ap.
The resistance change ratio ((Rxe2x80x2ap-Rxe2x80x2p)/Rxe2x80x2p) upon transition from the state of FIG. 6B to the state of FIG. 6A is smaller than the resistance change ratio (Rap-Rp)/Rp) upon transition from the state of FIG. 4C to the state of FIG. 4A. As a result, there is raised a serious problem that the TMR (change) ratio is considerably lowered.
As to such a problem, a magneto-resistive tunnel junction head has been proposed in Japanese patent application No. H11-171869, which can prevent a phenomenon wherein the current does not mainly flow at the antiparallel center portions of the layers but branches to mainly flow at both end portions where the resistance is low (the present inventors call this phenomenon xe2x80x9cextra current channel effectxe2x80x9d or xe2x80x9cthree current channel effectxe2x80x9d), so as to achieve a high head output for adaptation to ultrahigh density recording with less reduction in TMR ratio. Specifically, there has been a proposal for a magneto-resistive tunnel junction head having a tunnel multilayered film composed of a tunnel barrier layer, and a ferromagnetic free layer and a ferromagnetic pinned layer formed to sandwich the tunnel barrier layer therebetween, wherein a length of the ferromagnetic free layer in the longitudinal direction (bias magnetic field applying direction) thereof is set to be greater than a longitudinal length of the ferromagnetic pinned layer such that the ferromagnetic free layer is provided at the longitudinal opposite ends thereof with extended portions extending further beyond longitudinal opposite ends of the ferromagnetic pinned layer.
Under the circumstances, a further proposal has been demanded which can further improve a head characteristic of a magnet-resistive tunnel junction head having an element with a particular design such that a length of a ferromagnetic free layer in the longitudinal direction (bias magnetic field applying direction) thereof is set to be greater than a longitudinal length of a ferromagnetic pinned layer.
For solving the foregoing problems, according to one aspect of the present invention, there is provided a method of producing a magneto-resistive tunnel junction head comprising a tunnel multilayered film having a tunnel barrier layer, a ferromagnetic free layer and a ferromagnetic pinned layer such that the tunnel barrier layer is held between the ferromagnetic free layer and the ferromagnetic pinned layer, the method comprising a laminating step of forming the tunnel barrier layer and a non-magnetic metal protect layer in turn on the ferromagnetic pinned layer; an insulating layer forming step of forming side insulating layers on both sides of a lamination body having the ferromagnetic pinned layer, the tunnel barrier layer and the non-magnetic metal protect layer; a cleaning step of cleaning the surface of the non-magnetic metal protect layer; and a ferromagnetic free layer forming step of forming the ferromagnetic free layer such that the ferromagnetic free layer faces the ferromagnetic pinned layer via the cleaned surface.
According to another aspect of the present invention, there is provided a method of producing a magneto-resistive tunnel junction head comprising a tunnel multilayered film having a tunnel barrier layer, a ferromagnetic free layer and a ferromagnetic pinned layer such that the tunnel barrier layer is held between the ferromagnetic free layer and the ferromagnetic pinned layer, wherein the ferromagnetic free layer is applied with a bias magnetic field in a longitudinal direction thereof by biasing means disposed at and connected to longitudinal opposite ends thereof, and wherein a length of the ferromagnetic free layer in the longitudinal direction (bias magnetic field applying direction) thereof is set to be greater than a longitudinal length of the ferromagnetic pinned layer such that the ferromagnetic free layer is provided at the longitudinal opposite ends thereof with extended portions extending further beyond longitudinal opposite ends of the ferromagnetic pinned layer, the method comprising a laminating step of forming the tunnel barrier layer and a non-magnetic metal protect layer in turn on the ferromagnetic pinned layer; an insulating layer forming step of forming side insulating layers on both sides of a lamination body having the ferromagnetic pinned layer, the tunnel barrier layer and the non-magnetic metal protect layer; a cleaning step of cleaning the surface of the non-magnetic metal protect layer; a ferromagnetic free layer forming step of forming the ferromagnetic free layer such that the ferromagnetic free layer faces the ferromagnetic pinned layer via the cleaned surface; and a biasing means forming step of providing the biasing means such that the biasing means are disposed at and connected to both longitudinal opposite ends of the ferromagnetic free layer.
It is preferable that the cleaning step is carried out with a dry etching technique until the non-magnetic metal protect layer is completely removed.
It is preferable that the cleaning step is carried out with a dry etching technique until portions of the non-magnetic metal protect layer remain like islands.
It is preferable that the non-magnetic metal protect layer is made of at least one selected from Cu, Ag, Au and Al.
It is preferable that the thickness of the non-magnetic metal protect layer is set to be in the range of 20 to 100 xc3x85.
It is preferable that the biasing means located at the longitudinal opposite ends of the ferromagnetic free layer are contacted with upper or lower portions of the extended portions located at the longitudinal opposite ends of the ferromagnetic free layer, and that each of the biasing means is located with a predetermined space (D) from corresponding one of the longitudinal opposite ends of the ferromagnetic pinned layer, the space (D) being set to no less than 0.02 xcexcm.
It is preferable that the space (D) is set to no less than 0.02 xcexcm and no greater than 0.3 xcexcm.
It is preferable that the space (D) is set to no less than 0.02 xcexcm and less than 0.15 xcexcm.
It is preferable that a pinning layer for pinning magnetization of the ferromagnetic pinned layer is stacked on a surface of the ferromagnetic pinned layer remote from a side thereof abutting the tunnel barrier layer.
It is preferable that the tunnel multilayered film is electrically contacted with a pair of electrodes which are disposed to sandwich the tunnel multilayered film therebetween.
It is preferable that a pair of shield layers are formed to sandwich the pair of electrodes therebetween.